This invention relates generally to combustible cartridges and more particularly to an improved case construction utilizing threaded case components to form an improved metal head to case joint.
Combustible cases for large caliber ammunition have been used for a number of years. The advantage of using such cases is especially apparent in tank ammunition where disposal of spent cartridge cases is constrained by the cramped confines within the fighting vehicle. The combustible cases used in such ammunition are consumed during propellant ignition, leaving behind only the cartridge head containing the spent primer. Storage of spent cartridge heads measuring about three inches in length and five inches in diameter is much more convenient than wrestling with two foot hot long metal cases.
Typical combustible cartridge cases have a tubular body made of a cardboard type mixture such as wood fiber, kraft cellulose and nitrocellulose formed by the conventional beater additive or post impregnation processes. The outer surface of the body is impregnated with a plastic resin to protect the case from humidity and abrasion damage and add strength to the case.
The rear end of the tubular body generally has a cup shape with a central hole for passage of a primer flash tube through the end of the case into the propellant charge. A metal base or head is secured to this end of the combustible case by a snap ring and circular retainer arrangement with the primer stub or the base itself as is disclosed and shown in U.S. Pat. Nos. 4,159,678 and 4,928,598 to Luther et al.
Alternatively, the metal head to combustible case joint when the rear end of the case is a generally straight sided tube rather than a cupped end may be a snap fit arrangement utilizing a circumferential lip on or near the bottom end of the case to engage a corresponding bulge around the rim of the head such as is described in U.S. Pat. No. 4,738,202.
Another conventional method of securing the metal head to the case body is to provide the rear end of the case with a circumferential sleeve of a reduced diameter and gluing the sleeve to the inside of the rim of the metal head as is shown in U.S. Pat. No. 4,444,115 to Romer et al. The glue joint has the disadvantage that different coefficients of expansion between the metal and the cellulose combustible case may cause cracking and separation of the joint.
The snap fit arrangement has the disadvantage of being a relatively weak joint, susceptable of being popped apart when the cartridge is dropped. The snap ring and retainer arrangement has the disadvantage that the case must have a cupped shape rear end and therefore the case cannot be loaded from the rear with stick propellant. It must be loaded from the forward end. The glue joint has the disadvantage that it is permanent. It cannot be disassembled without damage to the case.
Accordingly there is a need for a secure metal base to case joint arrangement that permits rear loading of the propellant charge and retains the strength and rigidity of conventional connection, is removable and is simple to install and remove.
It is therefore an object of the present invention to provide an improved connection between combustible case and the metal head.
It is another object of the present invention to provide an improved threaded connection between the metal cartridge head and the combustible case.